Memory cell that includes a carbon-based memory element and methods of forming the same

ABSTRACT

Memory cells, and methods of forming such memory cells, are provided that include a steering element coupled to a carbon-based reversible resistivity switching material. In particular embodiments, methods in accordance with this invention form a single layer of a carbon-based reversible resistance switching material above a substrate, wherein the single layer of carbon material has a thickness greater than about three monolayers of the carbon-based reversible resistance switching material, and prior to forming an additional layer above the carbon layer, thermally anneal the carbon layer. Other aspects are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/087,164, filed Aug. 7, 2008, “Methods And Apparatus For Forming Memory Cells Using Carbon Read Writable Materials,” which is hereby incorporated by reference herein in its entirety for all purposes.

TECHNICAL FIELD

This invention relates to non-volatile memories, and more particularly to a memory cell that includes a carbon-based memory element, and methods of forming the same.

BACKGROUND

Non-volatile memories formed from reversible resistance switching elements are known. For example, U.S. patent application Ser. No. 11/968,154, filed Dec. 31, 2007, titled “Memory Cell That Employs A Selectively Fabricated Carbon Nano-Tube Reversible Resistance Switching Element And Methods Of Forming The Same” (the “'154 Application”), which is hereby incorporated by reference herein in its entirety for all purposes, describes a rewriteable non-volatile memory cell that includes a diode coupled in series with a carbon-based reversible resistivity switching material.

However, fabricating memory devices from carbon-based materials is technically challenging, and improved methods of forming memory devices that employ carbon-based materials are desirable.

SUMMARY

In a first aspect of the invention, a method of forming a memory cell is provided, the method including forming a single layer of a carbon-based reversible resistance switching material above a substrate, wherein the single layer of carbon material has a thickness greater than about three monolayers of the carbon-based reversible resistance switching material, and prior to forming an additional layer above the carbon layer, thermally annealing the carbon layer.

In a second aspect of the invention, a method of forming a memory cell is provided, the method including forming a layer of a carbon-based reversible resistance switching material above a substrate, and prior to forming an additional layer above the carbon layer, treating the carbon layer with ultraviolet radiation.

In a third aspect of the invention, a method of forming a memory cell is provided, the method including forming a layer of a carbon-based reversible resistance switching material above a substrate, and prior to forming an additional layer above the carbon layer, thermally annealing the carbon layer and treating the carbon layer with ultraviolet radiation.

In a fourth aspect of the invention, a method of forming a memory cell is provide, the method including forming a layer of a carbon-based reversible resistance switching material above a substrate, and prior to forming an additional layer above the carbon layer, thermally annealing the carbon layer in an environment that includes a gas comprising any of nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, helium, or xenon.

Other features and aspects of the present invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention can be more clearly understood from the following detailed description considered in conjunction with the following drawings, in which the same reference numerals denote the same elements throughout, and in which:

FIG. 1 is a diagram of an exemplary memory cell in accordance with this invention;

FIG. 2A is a simplified perspective view of an exemplary memory cell in accordance with this invention;

FIG. 2B is a simplified perspective view of a portion of a first exemplary memory level formed from a plurality of the memory cells of FIG. 2A;

FIG. 2C is a simplified perspective view of a portion of a first exemplary three-dimensional memory array in accordance with this invention;

FIG. 2D is a simplified perspective view of a portion of a second exemplary three-dimensional memory array in accordance with this invention;

FIG. 3 is a cross-sectional view of an exemplary embodiment of a memory cell in accordance with this invention; and

FIGS. 4A-4H illustrate cross-sectional views of a portion of a substrate during an exemplary fabrication of a single memory level in accordance with this invention.

DETAILED DESCRIPTION

Carbon films such as amorphous carbon (“aC”) containing nanocrystalline graphene (referred to herein as “graphitic carbon”), graphene, graphite, carbon nano-tubes, amorphous diamond-like carbon (“DLC”) (described below), silicon carbide, boron carbide and other similar carbon-based materials may exhibit resistivity-switching behavior that may make such materials suitable for use in microelectronic non-volatile memories.

Indeed, some carbon-based materials have demonstrated reversible resistivity-switching memory properties on lab-scale devices with a 100× separation between ON and OFF states and mid-to-high range resistance changes. Such a separation between ON and OFF states renders carbon-based materials viable candidates for memory cells formed using the carbon materials in memory elements in series with steering elements, such as tunnel junctions, diodes, thin film transistors, or the like.

A carbon-based resistivity-switching material may be characterized by its ratio of forms of carbon-carbon bonding. Carbon typically bonds to carbon to form either an sp²-bond (a trigonal carbon-carbon double bond (“C═C”)) or an sp³-bond (a tetrahedral carbon-carbon single bond (“C—C”)). In each case, a ratio of sp²-bonds to sp³-bonds can be determined via Raman spectroscopy by evaluating the D and G bands. In some embodiments, the range of materials may include those having a ratio such as M_(y)N_(z) where M is the sp³ material and N is the sp² material and y and z are any fractional value from zero to 1 as long as y+z=1. To provide sufficient resistivity-switching behavior useful in a memory device, the carbon-based material should have a relatively high concentration of sp² graphene crystallinity. DLC tends to be sp³-hybridized, and to be amorphous with respect to long range order, and also has found to be switchable.

A carbon-based memory element may be formed by arranging a carbon-based material between two electrodes to form a metal-insulator-metal (“MIM”) structure. In such a configuration, the carbon-based material sandwiched between the two metal or otherwise conducting layers serves as a reversible resistance-switching element. A memory cell may then be formed by coupling the MIM structure in series with a steering element, such as a diode.

Attempts to integrate the carbon material using traditional semiconductor processing techniques, however, have proven technically challenging. For example, carbon material is typically deposited using plasma enhanced chemical vapor deposition (“PECVD”) process at temperatures of about 550° C. or lower. However, carbon-based materials formed by PECVD may contain greater than 15 (“at %”) hydrogen content, which has several disadvantages when used as a resistivity-switching material in memory cells.

First, the hydrogen content is thermally unstable. In particular, dissociation of hydrogen occurs at process temperatures greater than about 450° C. When hydrogen dissociates, the structure of the carbon material changes, and the resistivity of the memory cell will vary. Indeed, memory cells that include carbon-based material containing high hydrogen content exhibit a broad distribution of resistivity. Second, a high hydrogen content hinders formation of sp² clusters in the carbon material. Consequently, a reduction of hydrogen will facilitate ordered sp² cluster formation. Third, a high hydrogen content in a carbon-based material can reduce material reliability, and increase OFF-state current. Thus, it is desirable to provide methods for reducing hydrogen content in carbon-based material used is memory cells.

Exemplary methods in accordance with this invention use post-deposition processing steps to reduce hydrogen content in the carbon-based material, which may form more ordered nanographitic carbon-based material. In particular, in an exemplary embodiment in accordance with this invention, a single layer of carbon-based material is formed having a thickness greater than about three monolayers of the carbon-based material. As used herein, a monolayer of a carbon-based material is about one atomic layer of the carbon-based material. Following formation of the single carbon material layer, and prior to forming additional layers above the carbon material layer, the carbon material layer is thermally annealed. During thermal annealing, the carbon material layer optionally may be doped with another element, such as nitrogen, argon, hydrogen, helium, xenon, carbon monoxide, carbon dioxide, or other similar element or combination of elements.

In an alternative exemplary embodiment in accordance with this invention, a single layer of carbon-based material is formed, and prior to forming additional layers above the carbon material layer, the carbon material layer is thermally annealed and doped with another element, such as nitrogen, argon, hydrogen, helium, xenon, carbon monoxide, carbon dioxide, or other similar element or combination of elements.

In still another alternative exemplary embodiment of this invention, following formation of a single layer of carbon material, and prior to forming additional layers above the carbon material layer, the carbon material layer is treated with UV radiation. Such UV treatment may greatly reduce overall thermal budget by the carbon material. During UV treatment, the carbon material layer optionally may be doped with another element, such as nitrogen, argon, hydrogen, helium, xenon, carbon monoxide, carbon dioxide, or other similar element or combination of elements.

In yet another alternative exemplary embodiment of this invention, following formation of a single layer of carbon material, and prior to forming additional layers above the carbon material layer, the carbon material layer is treated with UV radiation and then thermally annealed. In this combined embodiment, the UV treatment may supply energy to disrupt unstable bonds, and may therefore break carbon-hydrogen bonds and reduce hydrogen content in the carbon material. The subsequent thermal annealing step may supply thermal energy to form more ordered graphitic structure. During UV treatment and/or thermal annealing, the carbon material layer optionally may be doped with another element, such as nitrogen, argon, hydrogen, helium, xenon, carbon monoxide, carbon dioxide, or other similar element or combination of elements.

Subjecting carbon-based material to a thermal anneal treatment, and/or exposing the material to a UV treatment may reduce hydrogen content, and may improve material reliability, reduce off-state current, and/or promote sp² graphene formation and sp² graphene crystallization. Also, doping the carbon material layer (e.g., with nitrogen) may further promote sp² graphene crystallinity formation. Increasing sp² graphene crystallinity in a carbon-based material may improve the material's switching behavior and may enhance its use as a memory element. Higher crystallinity within a carbon-based material may facilitate better electrical performance, and may be associated with a higher ON-OFF ratio.

Exemplary Inventive Memory Cell

FIG. 1 is a schematic illustration of an exemplary memory cell 10 in accordance with this invention. Memory cell 10 includes a carbon-based reversible resistance-switching element 12 coupled to a steering element 14. Carbon-based reversible resistance-switching element 12 includes a carbon-based reversible resistivity-switching material (not separately shown) having a resistivity that may be reversibly switched between two or more states.

For example, carbon-based reversible resistivity-switching material of element 12 may be in an initial, low-resistivity state upon fabrication. Upon application of a first voltage and/or current, the material is switchable to a high-resistivity state. Application of a second voltage and/or current may return reversible resistivity-switching material to a low-resistivity state. Alternatively, carbon-based reversible resistance-switching element 12 may be in an initial, high-resistance state upon fabrication that is reversibly switchable to a low-resistance state upon application of the appropriate voltage(s) and/or current(s). When used in a memory cell, one resistance state may represent a binary “0,” whereas another resistance state may represent a binary “1,” although more than two data/resistance states may be used. Numerous reversible resistivity-switching materials and operation of memory cells employing reversible resistance switching elements are described, for example, in U.S. patent application Ser. No. 11/125,939, filed May 9, 2005 and titled “Rewriteable Memory Cell Comprising A Diode And A Resistance Switching Material” (the “'939 Application”), which is hereby incorporated by reference herein in its entirety for all purposes.

Steering element 14 may include a thin film transistor, a diode, metal-insulator-metal tunneling current device, or another similar steering element that exhibits non-ohmic conduction by selectively limiting the voltage across and/or the current flow through carbon-based reversible resistance-switching element 12. In this manner, memory cell 10 may be used as part of a two or three dimensional memory array and data may be written to and/or read from memory cell 10 without affecting the state of other memory cells in the array.

Exemplary embodiments of memory cell 10, carbon-based reversible resistance-switching element 12 and steering element 14 are described below with reference to FIGS. 2A-2D and FIG. 3.

Exemplary Embodiments of Memory Cells and Memory Arrays

FIG. 2A is a simplified perspective view of an exemplary embodiment of a memory cell 10 in accordance with this invention. Memory cell 10 includes a pillar 11 coupled between a first conductor 20 and a second conductor 22. Pillar 11 includes a carbon-based reversible resistance-switching element 12 coupled in series with a steering element 14. In some embodiments, a barrier layer 24 may be formed between carbon-based reversible resistance-switching element 12 and steering element 14, a barrier layer 28 may be formed between steering element 14 and first conductor 20, and a barrier layer 33 may be formed between carbon-based reversible resistance-switching element 12 and a metal layer 35. Barrier layers 24, 28 and 33 may include titanium nitride, tantalum nitride, tungsten nitride, or other similar barrier layer. In some embodiments, barrier layer 33 and metal layer 35 may be formed as part of upper conductor 22.

Carbon-based reversible resistance-switching element 12 may include a carbon-based material suitable for use in a memory cell. In exemplary embodiments of this invention, carbon-based reversible resistance-switching element 12 may include graphitic carbon. For example, in some embodiments, graphitic carbon reversible resistivity switching materials may be formed as described in U.S. patent application Ser. No. 12/499,467, filed Jul. 8, 2009 and titled “Carbon-Based Resistivity-Switching Materials And Methods Of Forming The Same” (the “'467 application”) (Docket No. SD-MXA-294), which is hereby incorporated by reference herein in its entirety for all purposes. In other embodiments, carbon-based reversible resistance-switching element 12 may include other carbon-based materials such as graphene, graphite, carbon nano-tube materials, DLC, silicon carbide, boron carbide, or other similar carbon-based materials. For simplicity, carbon-based reversible resistance-switching element 12 will be referred to in the remaining discussion interchangeably as “carbon element 12,” or “carbon layer 12.”

In an exemplary embodiment of this invention, steering element 14 includes a diode. In this discussion, steering element 14 is sometimes referred to as “diode 14.” Diode 14 may include any suitable diode such as a vertical polycrystalline p-n or p-i-n diode, whether upward pointing with an n-region above a p-region of the diode or downward pointing with a p-region above an n-region of the diode. For example, diode 14 may include a heavily doped n+ polysilicon region 14 a, a lightly doped or an intrinsic (unintentionally doped) polysilicon region 14 b above the n+ polysilicon region 14 a, and a heavily doped p+ polysilicon region 14 c above intrinsic region 14 b. It will be understood that the locations of the n+ and p+ regions may be reversed.

First conductor 20 and/or second conductor 22 may include any suitable conductive material such as tungsten, any appropriate metal, heavily doped semiconductor material, a conductive silicide, a conductive silicide-germanide, a conductive germanide, or the like. In the embodiment of FIG. 2A, first and second conductors 20 and 22, respectively, are rail-shaped and extend in different directions (e.g., substantially perpendicular to one another). Other conductor shapes and/or configurations may be used. In some embodiments, barrier layers, adhesion layers, antireflection coatings and/or the like (not shown) may be used with the first conductor 20 and/or second conductor 22 to improve device performance and/or aid in device fabrication.

FIG. 2B is a simplified perspective view of a portion of a first memory level 30 formed from a plurality of memory cells 10, such as memory cell 10 of FIG. 2A. For simplicity, carbon element 12, diode 14, barrier layers 24, 28 and 33, and metal layer 35 are not separately shown. Memory array 30 is a “cross-point” array including a plurality of bit lines (second conductors 22) and word lines (first conductors 20) to which multiple memory cells are coupled (as shown). Other memory array configurations may be used, as may multiple levels of memory.

For example, FIG. 2C is a simplified perspective view of a portion of a monolithic three dimensional array 40 a that includes a first memory level 42 positioned below a second memory level 44. Memory levels 42 and 44 each include a plurality of memory cells 10 in a cross-point array. Persons of ordinary skill in the art will understand that additional layers (e.g., an interlevel dielectric) may be present between the first and second memory levels 42 and 44, but are not shown in FIG. 2C for simplicity. Other memory array configurations may be used, as may additional levels of memory. In the embodiment of FIG. 2C, all diodes may “point” in the same direction, such as upward or downward depending on whether p-i-n diodes having a p-doped region on the bottom or top of the diodes are employed, simplifying diode fabrication.

For example, in some embodiments, the memory levels may be formed as described in U.S. Pat. No. 6,952,030, titled “High-Density Three-Dimensional Memory Cell,” which is hereby incorporated by reference herein in its entirety for all purposes. For instance, the upper conductors of a first memory level may be used as the lower conductors of a second memory level that is positioned above the first memory level as shown in FIG. 2D. In such embodiments, the diodes on adjacent memory levels preferably point in opposite directions as described in U.S. patent application Ser. No. 11/692,151, filed Mar. 27, 2007 and titled “Large Array Of Upward Pointing P-I-N Diodes Having Large And Uniform Current” (the “'151 Application”), which is hereby incorporated by reference herein in its entirety for all purposes. For example, as shown in FIG. 2D, the diodes of the first memory level 42 may be upward pointing diodes as indicated by arrow D1 (e.g., with p regions at the bottom of the diodes), whereas the diodes of the second memory level 44 may be downward pointing diodes as indicated by arrow D2 (e.g., with n regions at the bottom of the diodes), or vice versa.

A monolithic three dimensional memory array is one in which multiple memory levels are formed above a single substrate, such as a wafer, with no intervening substrates. The layers forming one memory level are deposited or grown directly over the layers of an existing level or levels. In contrast, stacked memories have been constructed by forming memory levels on separate substrates and adhering the memory levels atop each other, as in Leedy, U.S. Pat. No. 5,915,167, titled “Three Dimensional Structure Memory.” The substrates may be thinned or removed from the memory levels before bonding, but as the memory levels are initially formed over separate substrates, such memories are not true monolithic three dimensional memory arrays.

FIG. 3 is a cross-sectional view of an exemplary embodiment of memory cell 10 of FIG. 2A formed on a substrate, such as a wafer (not shown). In particular, memory cell 10 includes a pillar 11 coupled between first and second conductors 20 and 22, respectively. Pillar 11 includes carbon element 12 coupled in series with diode 14, and also may include barrier layers 24, 28, and 33, a silicide layer 50, a silicide-forming metal layer 52, and a metal layer 35. A dielectric layer 58 substantially surrounds pillar 11. In some embodiments, a sidewall liner 54 separates selected layers of pillar 11 from dielectric layer 58. Adhesion layers, antireflective coating layers and/or the like (not shown) may be used with first and/or second conductors 20 and 22, respectively, to improve device performance and/or facilitate device fabrication.

First conductor 20 may include any suitable conductive material such as tungsten, any appropriate metal, heavily doped semiconductor material, a conductive silicide, a conductive silicide-germanide, a conductive germanide, or the like. Second conductor 22 includes a barrier layer 26, which may include titanium nitride or other similar barrier layer material, and conductive layer 140, which may include any suitable conductive material such as tungsten, any appropriate metal, heavily doped semiconductor material, a conductive silicide, a conductive silicide-germanide, a conductive germanide, or the like.

Diode 14 may be a vertical p-n or p-i-n diode, which may either point upward or downward. In the embodiment of FIG. 2D in which adjacent memory levels share conductors, adjacent memory levels preferably have diodes that point in opposite directions such as downward-pointing p-i-n diodes for a first memory level and upward-pointing p-i-n diodes for an adjacent, second memory level (or vice versa).

In some embodiments, diode 14 may be formed from a polycrystalline semiconductor material such as polysilicon, a polycrystalline silicon-germanium alloy, polygermanium or any other suitable material. For example, diode 14 may include a heavily doped n+ polysilicon region 14 a, a lightly doped or an intrinsic (unintentionally doped) polysilicon region 14 b above the n+ polysilicon region 14 a, and a heavily doped p+ polysilicon region 14 c above intrinsic region 14 b. It will be understood that the locations of the n+ and p+ regions may be reversed.

In some embodiments, a thin germanium and/or silicon-germanium alloy layer (not shown) may be formed on n+ polysilicon region 14 a to prevent and/or reduce dopant migration from n+ polysilicon region 14 a into intrinsic region 14 b. Use of such a layer is described, for example, in U.S. patent application Ser. No. 11/298,331, filed Dec. 9, 2005 and titled “Deposited Semiconductor Structure To Minimize N-Type Dopant Diffusion And Method Of Making” (the “'331 Application”), which is hereby incorporated by reference herein in its entirety for all purposes. In some embodiments, a few hundred angstroms or less of silicon-germanium alloy with about 10 at % or more of germanium may be employed.

A barrier layer 28, such as titanium nitride, tantalum nitride, tungsten nitride, or other similar barrier layer material, may be formed between the first conductor 20 and the n+ region 14 a (e.g., to prevent and/or reduce migration of metal atoms into the polysilicon regions).

If diode 14 is fabricated from deposited silicon (e.g., amorphous or polycrystalline), a silicide layer 50 may be formed on diode 14 to place the deposited silicon in a low resistivity state, as fabricated. Such a low resistivity state allows for easier programming of memory cell 10, as a large voltage is not required to switch the deposited silicon to a low resistivity state. For example, a silicide-forming metal layer 52 such as titanium or cobalt may be deposited on p+ polysilicon region 14 c. In some embodiments, an additional nitride layer (not shown) may be formed at a top surface of silicide-forming metal layer 52. In particular, for highly reactive metals, such as titanium, an additional cap layer such as TiN layer may be formed on silicide-forming metal layer 52. Thus, in such embodiments, a Ti/TiN stack is formed on top of p+ polysilicon region 14 c.

A rapid thermal anneal (“RTA”) step may then be performed to form silicide regions by reaction of silicide-forming metal layer 52 with p+ region 14 c. The RTA step may be performed at a temperature between about 650° C. to about 750° C., more generally between about 600° C. to about 800° C., preferably at about 750° C., for a duration between about 10 seconds to about 60 seconds, more generally between about 10 seconds to about 90 seconds, preferably about 1 minute, and causes silicide-forming metal layer 52 and the deposited silicon of diode 14 to interact to form silicide layer 50, consuming all or a portion of the silicide-forming metal layer 52.

As described in U.S. Pat. No. 7,176,064, titled “Memory Cell Comprising A Semiconductor Junction Diode Crystallized Adjacent To A Silicide,” which is hereby incorporated by reference herein in its entirety for all purposes, silicide-forming materials such as titanium and/or cobalt react with deposited silicon during annealing to form a silicide layer. The lattice spacing of titanium silicide and cobalt silicide are close to that of silicon, and it appears that such silicide layers may serve as “crystallization templates” or “seeds” for adjacent deposited silicon as the deposited silicon crystallizes (e.g., silicide layer 50 enhances the crystalline structure of silicon diode 14 during annealing). Lower resistivity silicon thereby is provided. Similar results may be achieved for silicon-germanium alloy and/or germanium diodes.

In embodiments in which a nitride layer was formed at a top surface of silicide-forming metal layer 52, following the RTA step, the nitride layer may be stripped using a wet chemistry. For example, if silicide-forming metal layer 52 includes a TiN top layer, a wet chemistry (e.g., ammonium, peroxide, water in a 1:1:1 ratio) may be used to strip any residual TiN.

A barrier layer 33, such as titanium nitride, tantalum nitride, tungsten nitride, or other similar barrier layer material, may be formed above carbon layer 12.

As discussed above, carbon-based materials formed by PECVD techniques may contain greater than 15 at % hydrogen content, which has several disadvantages when used as a resistivity-switching material in memory cells. Hydrogen content in a carbon-based material can reduce material reliability, increase off-state current, and hinder sp² graphene crystallinity formation.

Methods in accordance with this invention seek to reduce hydrogen content in carbon layer 12 by forming carbon layer 12, and then using various post-carbon-deposition treatment processes to remove hydrogen from the deposited carbon material. The exemplary post-carbon-deposition treatment processes preferably occur after the carbon layer 12 has been formed, but before additional layers are formed above carbon layer 12. In particular, exemplary methods in accordance with this invention form carbon layer 12, and then treat the deposited carbon layer 12 by thermally annealing carbon layer 12, exposing carbon layer 12 to ultraviolet (“UV”) radiation, or UV irradiating and then thermally annealing carbon layer 12. In addition, during the thermal annealing and/or the UV irradiation steps, carbon layer 12 may be doped with another element, such as such as nitrogen, argon, hydrogen, oxygen, helium, xenon, or other similar element. Each of these exemplary processes will be discussed in turn.

Thermal Annealing

In a first exemplary method of this invention, carbon layer 12 is formed as a single layer of carbon-based material having a thickness between about three monolayers to about five monolayers of the carbon-based material. After forming the single layer of carbon-based material, carbon layer 12 is then thermally annealed, preferably before additional material layers are formed on carbon layer 12. This process is iteratively repeated, until carbon layer 12 has a desired thickness. In some embodiments of this invention, carbon layer 12 may be formed having a desired thickness between about 100 and about 600 angstroms, more generally between about 1 and about 1000 angstroms. Other thicknesses may be used.

Thermal annealing can be done by two different methods: conventional thermal annealing or RTA. In conventional thermal annealing, carbon layer 12 is thermally heated at a ramp-up rate between about 5° C./sec to about 30° C./sec, and is cooled at a ramp down rate between about 5° C./sec to about 30° C./sec. The anneal may be performed for about 5 minutes to about 120 minutes, at a temperature of between about 540° C. and about 750° C. More generally, the thermal anneal may be performed for about 1 minute to about 300 minutes, at a temperature of between about 500° C. and about 1200° C. Other annealing times, temperatures and ramp up/down rates may be used.

Alternatively, in RTA, carbon layer 12 is thermally heated at a ramp-up rate between about 10° C./sec to about 100° C./sec, and is cooled at a ramp down rate between about 10° C./sec to about 100° C./sec. The anneal may be performed for about 60 seconds to about 10 minutes, at a temperature of between about 540° C. and about 750° C. More generally, the thermal anneal may be performed for about 60 seconds to about 20 minutes, at a temperature of between about 500° C. and about 1000° C.

The thermal annealing (conventional or RTA) may performed in vacuum. Alternatively, thermal annealing optionally may be performed in an environment that includes that includes a gas comprising any of N₂, Ar, H₂, carbon monoxide, carbon dioxide, or other similar gas, or He, Xe, or other inert gas, at gas flow rates ranging from about 1000 to about 5000 standard cubic centimeters per minute (“sccm”). In particular, such treatment may facilitate doping carbon material layer 12 with another element, such as such as nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, helium, xenon, or other similar element. For example, to dope carbon layer 12 with nitrogen, the thermal anneal may be performed in a nitrogen source, such as ammonia, nitrous oxide, or other similar source of nitrogen.

In an alternative exemplary method of this invention, carbon layer 12 is formed as a single layer of carbon-based material, and then carbon layer 12 is thermally annealed and doped with another element, such by performing the anneal in a nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, helium, xenon, or other similar source. The thermal anneal/doping preferably is performed before additional material layers are formed on carbon layer 12. The exemplary times, temperatures and queue/ramp times specified above may be used.

Although not wanting to be bound to any particular theory, it is believed that conventional thermal annealing may supply thermal energy to form more ordered graphitic structure in carbon layer 12. In contrast, it is believed that RTA may break carbon-hydrogen bonds and reduce hydrogen content in the carbon material. Removal of hydrogen may further facilitate carbon-carbon bond formation. Also, doping (e.g., with nitrogen or other element) may promote sp² graphene crystallinity formation.

UV Irradiation

In accordance with a second exemplary method of this invention, carbon layer 12 is formed, and the deposited carbon layer 12 is then exposed to UV radiation, preferably before additional material layers are formed on carbon layer 12. In some embodiments of this invention, carbon layer 12 may be formed having a thickness between about 200 and about 800 angstroms, more generally between about 1 and about 1000 angstroms. Other thicknesses may be used. The wavelength of UV irradiation may be selected to optimize absorption by carbon-hydrogen bonds in carbon layer 12, which may facilitate disruption of carbon-hydrogen bonds and their replacement by more desirable stable carbon-carbon bonds.

In exemplary embodiments of this invention, the wafer is held at a temperature between about 25° C. to about 250° C., and UV treatment is performed for about 10 seconds to about 15 minutes, at wavelengths between about 230 nanometers to about 300 nanometers, and at a power of between about 1 μW/cm² to about 2000 mW/cm². More generally, the UV treatment may be performed for about 1 second to about 60 minutes, at wavelengths of between about 230 nanometers to about 400 nanometers, and at a power of between about 1 μW/cm² to about 3000 mW/cm². The UV treatment may be performed under vacuum, or optionally may be performed at a pressure between about 30 mT to about 760 T, with at least one of N₂, Ar, H₂, CO, CO₂, or other similar gas, or He, Xe, or other inert gas, at gas flow rates ranging from about 100 sccm to about 5000 sccm.

During the UV treatment, carbon material layer 12 optionally may be doped with another element, such as such as nitrogen, oxygen, or other similar element. For example, to dope carbon layer 12 with nitrogen, the UV treatment may be performed in a nitrogen source, such as ammonia, nitrous oxide, or other similar source of nitrogen.

Although not wanting to be bound to any particular theory, it is believed that exposing carbon layer 12 to UV radiation at wavelengths less than about 400 nanometers may cause dissociation of CH bonds in the material and may facilitate hydrogen removal from carbon layer 12. As previously mentioned, doping (e.g., with nitrogen) may further promote sp² graphene crystallinity formation.

UV Irradiation and Thermal Annealing

In accordance with a third exemplary embodiment of this invention, carbon layer 12 is formed, and the deposited carbon layer 12 is then UV treated and then thermally annealed, preferably before additional material layers are formed on carbon layer 12. In some embodiments of this invention, carbon layer 12 may be formed having a thickness between about 200 and about 800 angstroms, more generally between about 1 and about 1000 angstroms. Other thicknesses may be used. As in the other exemplary methods, carbon layer 12 also may be doped during the UV treatment and/or the thermal anneal.

Exemplary Fabrication Processes for Memory Cells

Referring now to FIGS. 4A-4H, a first exemplary method of forming an exemplary memory level in accordance with this invention is described. In particular, FIGS. 4A-4H illustrate an exemplary method of forming an exemplary memory level including memory cells 10 of FIG. 3. As will be described below, the first memory level includes a plurality of memory cells that each include a steering element and a carbon-based reversible resistance switching element coupled to the steering element. Additional memory levels may be fabricated above the first memory level (as described previously with reference to FIGS. 2C-2D).

With reference to FIG. 4A, substrate 100 is shown as having already undergone several processing steps. Substrate 100 may be any suitable substrate such as a silicon, germanium, silicon-germanium, undoped, doped, bulk, silicon-on-insulator (“SOI”) or other substrate with or without additional circuitry. For example, substrate 100 may include one or more n-well or p-well regions (not shown).

Isolation layer 102 is formed above substrate 100. In some embodiments, isolation layer 102 may be a layer of silicon dioxide, silicon nitride, silicon oxynitride or any other similar insulating layer.

Following formation of isolation layer 102, an adhesion layer 104 is formed over isolation layer 102 (e.g., by physical vapor deposition (“PVD”) or other similar method). For example, adhesion layer 104 may be about 20 to about 500 angstroms, and preferably about 100 angstroms, of titanium nitride or another suitable adhesion layer such as tantalum nitride, tungsten nitride, combinations of one or more adhesion layers, or the like. Other adhesion layer materials and/or thicknesses may be employed. In some embodiments, adhesion layer 104 may be optional.

After formation of adhesion layer 104, a conductive layer 106 is deposited over adhesion layer 104. Conductive layer 106 may include any suitable conductive material such as tungsten or another appropriate metal, heavily doped semiconductor material, a conductive silicide, a conductive silicide-germanide, a conductive germanide, or the like deposited by any suitable method (e.g., chemical vapor deposition (“CVD”), PVD, etc.). In at least one embodiment, conductive layer 106 may comprise about 200 to about 2500 angstroms of tungsten. Other conductive layer materials and/or thicknesses may be used.

Following formation of conductive layer 106, adhesion layer 104 and conductive layer 106 are patterned and etched. For example, adhesion layer 104 and conductive layer 106 may be patterned and etched using conventional lithography techniques, with a soft or hard mask, and wet or dry etch processing. In at least one embodiment, adhesion layer 104 and conductive layer 106 are patterned and etched to form substantially parallel, substantially co-planar first conductors 20. Exemplary widths for first conductors 20 and/or spacings between first conductors 20 range from about 200 to about 2500 angstroms, although other conductor widths and/or spacings may be used.

After first conductors 20 have been formed, a dielectric layer 58 a is formed over substrate 100 to fill the voids between first conductors 20. For example, approximately 3000-7000 angstroms of silicon dioxide may be deposited on the substrate 100 and planarized using chemical mechanical polishing or an etchback process to form a planar surface 110. Planar surface 110 includes exposed top surfaces of first conductors 20 separated by dielectric material (as shown). Other dielectric materials such as silicon nitride, silicon oxynitride, low K dielectrics, etc., and/or other dielectric layer thicknesses may be used. Exemplary low K dielectrics include carbon doped oxides, silicon carbon layers, or the like.

In other embodiments of the invention, first conductors 20 may be formed using a damascene process in which dielectric layer 58 a is formed, patterned and etched to create openings or voids for first conductors 20. The openings or voids then may be filled with adhesion layer 104 and conductive layer 106 (and/or a conductive seed, conductive fill and/or barrier layer if needed). Adhesion layer 104 and conductive layer 106 then may be planarized to form planar surface 110. In such an embodiment, adhesion layer 104 will line the bottom and sidewalls of each opening or void.

Following planarization, the diode structures of each memory cell are formed. With reference to FIG. 4B, a barrier layer 28 is formed over planarized top surface 110 of substrate 100. Barrier layer 28 may be about 20 to about 500 angstroms, and preferably about 100 angstroms, of titanium nitride or another suitable barrier layer such as tantalum nitride, tungsten nitride, combinations of one or more barrier layers, barrier layers in combination with other layers such as titanium/titanium nitride, tantalum/tantalum nitride or tungsten/tungsten nitride stacks, or the like. Other barrier layer materials and/or thicknesses may be employed.

After deposition of barrier layer 28, deposition of the semiconductor material used to form the diode of each memory cell begins (e.g., diode 14 in FIGS. 2 and 3). Each diode may be a vertical p-n or p-i-n diode as previously described. In some embodiments, each diode is formed from a polycrystalline semiconductor material such as polysilicon, a polycrystalline silicon-germanium alloy, polygermanium or any other suitable material. For convenience, formation of a polysilicon, downward-pointing diode is described herein. It will be understood that other materials and/or diode configurations may be used.

With reference to FIG. 4B, following formation of barrier layer 28, a heavily doped n+ silicon layer 14 a is deposited on barrier layer 28. In some embodiments, n+ silicon layer 14 a is in an amorphous state as deposited. In other embodiments, n+ silicon layer 14 a is in a polycrystalline state as deposited. CVD or another suitable process may be employed to deposit n+ silicon layer 14 a. In at least one embodiment, n+ silicon layer 14 a may be formed, for example, from about 100 to about 1000 angstroms, preferably about 100 angstroms, of phosphorus or arsenic doped silicon having a doping concentration of about 10²¹ cm⁻³. Other layer thicknesses, doping types and/or doping concentrations may be used. N+ silicon layer 14 a may be doped in situ, for example, by flowing a donor gas during deposition. Other doping methods may be used (e.g., implantation).

After deposition of n+ silicon layer 14 a, a lightly doped, intrinsic and/or unintentionally doped silicon layer 14 b may be formed over n+ silicon layer 14 a. In some embodiments, intrinsic silicon layer 14 b may be in an amorphous state as deposited. In other embodiments, intrinsic silicon layer 14 b may be in a polycrystalline state as deposited. CVD or another suitable deposition method may be employed to deposit intrinsic silicon layer 14 b. In at least one embodiment, intrinsic silicon layer 14 b may be about 500 to about 4800 angstroms, preferably about 2500 angstroms, in thickness. Other intrinsic layer thicknesses may be used.

A thin (e.g., a few hundred angstroms or less) germanium and/or silicon-germanium alloy layer (not shown) may be formed on n+ silicon layer 14 a prior to depositing intrinsic silicon layer 14 b to prevent and/or reduce dopant migration from n+ silicon layer 14 a into intrinsic silicon layer 14 b (as described in the '331 Application, previously incorporated).

Heavily doped, p-type silicon may be either deposited and doped by ion implantation or may be doped in situ during deposition to form a p+ silicon layer 14 c. For example, a blanket p+ implant may be employed to implant boron a predetermined depth within intrinsic silicon layer 14 b. Exemplary implantable molecular ions include BF₂, BF₃, B and the like. In some embodiments, an implant dose of about 1−5×10¹⁵ ions/cm² may be employed. Other implant species and/or doses may be used. Further, in some embodiments, a diffusion process may be employed. In at least one embodiment, the resultant p+ silicon layer 14 c has a thickness of about 100-700 angstroms, although other p+ silicon layer sizes may be used.

Following formation of p+ silicon layer 14 c, a silicide-forming metal layer 52 is deposited over p+ silicon layer 14 c. Exemplary silicide-forming metals include sputter or otherwise deposited titanium or cobalt. In some embodiments, silicide-forming metal layer 52 has a thickness of about 10 to about 200 angstroms, preferably about 20 to about 50 angstroms and more preferably about 20 angstroms. Other silicide-forming metal layer materials and/or thicknesses may be used. A nitride layer (not shown) may be formed at the top of silicide-forming metal layer 52.

Following formation of silicide-forming metal layer 52, an RTA step may be performed to form silicide layer 50, consuming all or a portion of the silicide-forming metal layer 52. The RTA step may be performed at a temperature between about 650° C. and about 750° C., more generally between about 600° C. and about 800° C., preferably at about 750° C., for a duration between about 10 seconds to about 60 seconds, more generally between about 10 seconds to about 90 seconds, preferably about 60 seconds. Following the RTA step, any residual nitride layer from silicide-forming metal layer 52 may be stripped using a wet chemistry, as described above, and as is known in the art.

Following the RTA step and the nitride strip step, a barrier layer 24 is deposited. Barrier layer 24 may be about 20 to about 500 angstroms, and preferably about 200 angstroms, of titanium nitride or another suitable barrier layer such as tantalum nitride, tungsten nitride, combinations of one or more barrier layers, barrier layers in combination with other layers such as titanium/titanium nitride, tantalum/tantalum nitride or tungsten/tungsten nitride stacks, or the like. Other barrier layer materials and/or thicknesses may be employed. Any suitable method may be used to form barrier layer 24. For example, PVD, atomic layer deposition (“ALD”), or the like may be used.

Next, carbon layer 12 is deposited over barrier layer 24. Carbon layer 12 may be formed by a PECVD method, for example. Other methods may be used, including, without limitation, sputter deposition from a target, PVD, CVD, arc discharge techniques, and laser ablation. Other methods may be used to form carbon layer 12, such as a damascene integration method, for example. Carbon layer 12 may include graphitic carbon. In alternative embodiments, other carbon-based materials may be used, such as graphene, graphite, carbon nano-tube materials, DLC or other similar carbon-based materials. Carbon layer 12 is formed having a thickness between about 100 and about 600 angstroms, more generally between about 1 and about 1000 angstroms. Other thicknesses may be used.

In accordance with this invention, and as described above in connection with the description of FIG. 3, carbon layer 12 is subjected to one or more post-carbon-deposition processes to treat deposited carbon layer 12, preferably before additional layers are formed above carbon layer 12. For example, as described above, exemplary methods in accordance with this invention treat the deposited carbon layer 12 by thermally annealing carbon layer 12, exposing carbon layer 12 to UV radiation, or UV irradiating and then thermally annealing carbon layer 12.

In an exemplary embodiment, carbon layer 12 is formed as a single layer of carbon-based material having a thickness between about three monolayers to about five monolayers of the carbon-based material. After forming the single layer of carbon-based material, carbon layer 12 is then thermally annealed, preferably before additional material layers are formed on carbon layer 12. This process is repeated, until carbon layer 12 has a desired thickness. In some embodiments of this invention, carbon layer 12 may be formed having a desired thickness between about 100 and about 600 angstroms, more generally between about 1 and about 1000 angstroms. During the thermal annealing process, carbon layer 12 may be doped with another element, such as such as nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, helium, xenon, or other similar element.

In an alternative exemplary method of this invention, carbon layer 12 is formed as a single layer of carbon-based material, and then carbon layer 12 is thermally annealed and doped with another element, such by performing the anneal in a nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, helium, xenon, or other similar source. The thermal anneal/doping preferably is performed before additional material layers are formed on carbon layer 12.

In another exemplary method of this invention, carbon layer 12 is formed, and the deposited carbon layer 12 is then exposed to UV radiation, preferably before additional material layers are formed on carbon layer 12. During the UV treatment, carbon material layer 12 optionally may be doped with another element, such as such as nitrogen, oxygen, or other similar element.

In still another exemplary embodiment, carbon layer 12 is formed, and the deposited carbon layer 12 is then UV treated and then thermally annealed, preferably before additional material layers are formed on carbon layer 12. As in the other exemplary methods, carbon layer 12 also may be doped during the UV treatment and/or the thermal anneal.

Referring again to FIG. 4B, barrier layer 33 is formed over carbon layer 12. Barrier layer 33 may be about 5 to about 800 angstroms, and preferably about 100 angstroms, of titanium nitride or another suitable barrier layer such as tantalum nitride, tungsten nitride, combinations of one or more barrier layers, barrier layers in combination with other layers such as titanium/titanium nitride, tantalum/tantalum nitride or tungsten/tungsten nitride stacks, or the like. Other barrier layer materials and/or thicknesses may be employed.

Next, a metal layer 35 may be deposited over barrier layer 33. For example, between about 800 to about 1200 angstroms, more generally between about 500 angstroms to about 1500 angstroms, of tungsten may be deposited on barrier layer 33. Other materials and thicknesses may be used. Any suitable method may be used to form metal layer 35. For example, CVD, PVD, or the like may be employed. As described in more detail below, metal layer 35 may be used as a hard mask layer, and also may be used as a stop during a subsequent chemical mechanical planarization (“CMP”) step. A hard mask is an etched layer which serves to pattern the etch of an underlying layer.

As shown in FIG. 4C, metal layer 35 is patterned and etched to form patterned metal hardmask regions 35. Patterned metal hardmask regions 35 may have about the same pitch and about the same width as conductors 20 below, such that each patterned metal hardmask regions 35 is formed on top of a conductor 20. Some misalignment may be tolerated. Persons of ordinary skill in the art will understand that patterned metal hardmask regions 35 may have a smaller width than conductors 20.

For example, photoresist (“PR”) may be deposited on metal layer 35, patterned using standard photolithography techniques, and then the photoresist may be removed. Alternatively, a hard mask of some other material, for example silicon dioxide, may be formed on top of metal layer 33, with bottom antireflective coating (“BARC”) on top, then patterned and etched. Similarly, dielectric antireflective coating (“DARC”) may be used as a hard mask.

As shown in FIG. 4D, metal hardmask regions 35 are used to pattern and etch barrier layer 33, carbon layer 12, silicide-forming metal layer 52, diode layers 14 a-14 c and barrier layer 28 to form pillars 132. Pillars 132 may have about the same pitch and about the same width as conductors 20 below, such that each pillar 132 is formed on top of a conductor 20. Some misalignment may be tolerated. Persons of ordinary skill in the art will understand that pillars 132 may have a smaller width than conductors 20.

Any suitable etch chemistries, and any suitable etch parameters, flow rates, chamber pressures, power levels, process temperatures, and/or etch rates may be used. In some embodiments, barrier layer 33, carbon nitride barrier layer 31, carbon element 12, barrier layer 24, silicide-forming metal layer 52, diode layers 14 a-14 c and barrier layer 28 may be patterned using a single etch step. In other embodiments, separate etch steps may be used. The etch proceeds down to dielectric layer 58 a.

In some exemplary embodiments, the memory cell layers may be etched using chemistries selected to minimize or avoid damage to carbon material. For example, O₂, CO, N₂, or H₂, or other similar chemistries may be used. In embodiments in which CNT material is used in the memory cells, oxygen (“O₂”), boron trichloride (“BCl₃”) and/or chlorine (“Cl₂”) chemistries, or other similar chemistries, may be used. Any suitable etch parameters, flow rates, chamber pressures, power levels, process temperatures, and/or etch rates may be used. Exemplary methods for etching carbon material are described, for example, in U.S. patent application Ser. No. 12/415,964, “Electronic Devices Including Carbon-Based Films Having Sidewall Liners, and Methods of Forming Such Devices,” filed Mar. 31, 2009 (Docket No. SD-MXA-315), which is hereby incorporated by reference in its entirety for all purposes.

After the memory cell layers have been etched, pillars 132 may be cleaned. In some embodiments, a dilute hydrofluoric/sulfuric acid clean is performed. Post-etch cleaning may be performed in any suitable cleaning tool, such as a Raider tool, available from Semitool of Kalispell, Mont. Exemplary post-etch cleaning may include using ultra-dilute sulfuric acid (e.g., about 1.5-1.8wt %) for about 60 seconds and ultra-dilute hydrofluoric (“HF”) acid (e.g., about 0.4-0.6 wt % ) for about 60 seconds. Megasonics may or may not be used. Alternatively, H₂SO₄ may be used.

After pillars 132 have been cleaned, an in-situ anneal or degas in vacuum step may be performed. Carbon material tends to absorb moisture, especially during a wet clean process. This is problematic, because trapped moisture may result in de-lamination of carbon material and degradation in switching. In-situ annealing or degas in vacuum helps to drive out moisture before the next process step. In particular, the in-situ anneal or degas in vacuum is performed in the chamber of the next processing step. Degas in vacuum can also be performed in a transfer chamber or loadlock mounted on the same platform as that process chamber. For example, if the next processing step is formation of a sidewall liner, the in-situ anneal is performed in the chamber used to form the sidewall liner. The in-situ anneal may be performed at a temperature between about 200° C. and about 350° C., more generally between about 200° C. and about 450° C., for a duration between about 1 to about 2 minutes, more generally between about 30 seconds and about 5 minutes, at a pressure of between about 0.1 mT to about 10 T, more generally between about 0.1 mT to about 760 T. Alternatively, the in-situ anneal may be performed in an environment containing Ar, He, or N₂, or a forming gas containing H₂ and N₂, at a flow rate of between about 1000 to about 8000 sccm, more generally between about 1000-20000 sccm. If degas in vacuum step is used instead of in-situ annealing, the degas is performed at a pressure between about 0.1 mT to about 50 mT, and at a temperature between about room temperature to about 450° C.

Next, a conformal dielectric liner 54 is deposited above and around pillars 132, resulting in the exemplary structure illustrated in FIG. 4E. Dielectric liner 54 may be formed with an oxygen-poor deposition chemistry (e.g., without a high density of oxygen plasma) to protect sidewalls of carbon layer 12 during a subsequent deposition containing a high oxygen plasma density of gap-fill dielectric 58 b (e.g., SiO₂) (not shown in FIG. 4E).

In an exemplary embodiment of this invention, dielectric liner 54 may be formed from boron nitride, such as described in commonly owned co-pending U.S. patent application Ser. No. 12/536,457, “A Memory Cell That Includes A Carbon-Based Memory Element And Methods Of Forming The Same,” filed Aug. 5, 2009 (Docket Number SD-MXA-335), which is incorporated by reference herein in its entirely for all purposes. Alternatively, dielectric sidewall liner 54 may be formed from other materials, such as SiN, Si_(x)C_(y)N_(z), Si_(x)O_(y)N_(z), Si_(x)B_(y)N_(z), (with low O content), where x, y and z are non-zero numbers resulting in stable compounds. Persons of ordinary skill in the art will understand that other dielectric materials may be used to form dielectric liner 54.

In one exemplary embodiment, a SiN dielectric sidewall liner 54 may be formed by PECVD using the process parameters listed in Table 1. Liner film thickness scales linearly with time. Other powers, temperatures, pressures, thicknesses and/or flow rates may be used.

TABLE 1 PECVD SiN LINER PROCESS PARAMETERS EXEMPLARY PROCESS PARAMETER RANGE PREFERRED RANGE SiH₄ Flow Rate (slm) 0.1-2.0 0.4-0.7 NH₃ Flow Rate (slm)  1-10 2-8 N₂ Flow Rate (slm) 0.5-10  1.0-5   Temperature (° C.) 300-500 350-450 Low Frequency Bias (kW) 0-1 0.2-0.6 High Frequency Bias (kW) 0-1 0.2-0.6 Thickness (Angstroms) 100-500 250-350

With reference to FIG. 4F, an anisotropic etch is used to remove lateral portions of sidewall liner 54, leaving only sidewall portions of sidewall liner 54 on the sides of pillars 132. For example, a sputter etch or other suitable process may be used to anisotropically etch sidewall liner 54. Dielectric sidewall liner 54 may protect the carbon material of carbon layer 12 from damage during deposition of dielectric layer 58 b (not shown in FIG. 4F), described below.

Next, a dielectric layer 58 b may be deposited over pillars 132 to fill the voids between pillars 132. For example, approximately 200-7000 angstroms of silicon dioxide may be deposited and planarized using CMP or an etchback process to remove excess dielectric material 58 b and form a planar surface 134, resulting in the structure illustrated in FIG. 4G. During the planarization process, metal hardmask regions 35 may be used as a CMP stop. Planar surface 134 includes exposed top surfaces of pillars 132 separated by dielectric material 58 b (as shown). Other dielectric materials such as silicon nitride, silicon oxynitride, low K dielectrics, etc., and/or other dielectric layer thicknesses may be used. Exemplary low K dielectrics include carbon doped oxides, silicon carbon layers, or the like.

With reference to FIG. 4H, second conductors 22 may be formed above pillars 132 in a manner similar to the formation of first conductors 20. For example, in some embodiments, one or more barrier layers and/or adhesion layers 26 may be deposited over pillars 132 prior to deposition of a conductive layer 140 used to form second conductors 22.

Conductive layer 140 may be formed from any suitable conductive material such as tungsten, another suitable metal, heavily doped semiconductor material, a conductive silicide, a conductive silicide-germanide, a conductive germanide, or the like deposited by any suitable method (e.g., CVD, PVD, etc.). Other conductive layer materials may be used. Barrier layers and/or adhesion layers 26 may include titanium nitride or another suitable layer such as tantalum nitride, tungsten nitride, combinations of one or more layers, or any other suitable material(s). The deposited conductive layer 140 and barrier and/or adhesion layer 26 may be patterned and etched to form second conductors 22. In at least one embodiment, second conductors 22 are substantially parallel, substantially coplanar conductors that extend in a different direction than first conductors 20.

In other embodiments of the invention, second conductors 22 may be formed using a damascene process in which a dielectric layer is formed, patterned and etched to create openings or voids for conductors 22. The openings or voids may be filled with adhesion layer 26 and conductive layer 140 (and/or a conductive seed, conductive fill and/or barrier layer if needed). Adhesion layer 26 and conductive layer 140 then may be planarized to form a planar surface.

Following formation of second conductors 22, the resultant structure may be annealed to crystallize the deposited semiconductor material of diodes 14 (and/or to form silicide regions by reaction of the silicide-forming metal layer 52 with p+ region 14 c). The lattice spacing of titanium silicide and cobalt silicide are close to that of silicon, and it appears that silicide layers 50 may serve as “crystallization templates” or “seeds” for adjacent deposited silicon as the deposited silicon crystallizes (e.g., silicide layer 50 enhances the crystalline structure of silicon diode 14 during annealing at temps of about 600-800° C.). Lower resistivity diode material thereby is provided. Similar results may be achieved for silicon-germanium alloy and/or germanium diodes.

Thus in at least one embodiment, a crystallization anneal may be performed for about 10 seconds to about 2 minutes in nitrogen at a temperature of about 600° C. to about 800° C., and more preferably between about 650° C. to about 750° C. Other annealing times, temperatures and/or environments may be used.

Persons of ordinary skill in the art will understand that alternative memory cells in accordance with this invention may be fabricated in other similar techniques. For example, memory cells may be formed that include carbon layer 12 below diode 14.

The foregoing description discloses only exemplary embodiments of the invention. Modifications of the above disclosed apparatus and methods which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, in any of the above embodiments, carbon layer 12 may be located below diodes 14. As stated, although the invention has been described primarily with reference to graphitic carbon, other carbon-based materials may be similarly used. Further, each carbon-based layer is preferably formed between two conducting layers such as titanium nitride or other barrier/adhesion layers to form a MIM stack in series with a steering element.

Accordingly, although the present invention has been disclosed in connection with exemplary embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims. 

1. A method of forming a memory cell, the method comprising: forming a single layer of a carbon-based reversible resistance switching material above a substrate, wherein the single layer of carbon material has a thickness greater than about three monolayers of the carbon-based reversible resistance switching material; and prior to forming an additional layer above the carbon layer, thermally annealing the carbon layer.
 2. The method of claim 1, wherein thermally annealing comprises thermally annealing at a temperature of between about 500° C. and about 1200° C.
 3. The method of claim 2, wherein thermally annealing comprises thermally annealing for about 1 minute to about 300 minutes.
 4. The method of claim 1, further comprising doping the carbon layer by performing the thermal anneal step in an environment that includes a gas comprising any of nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, helium, or xenon.
 5. The method of claim 1, wherein the carbon layer comprises amorphous carbon.
 6. The method of claim 5, wherein the amorphous carbon contains nanocrystalline graphene.
 7. The method of claim 1, further comprising iteratively repeating the forming and thermally annealing steps to form a carbon layer that has a thickness between about 1 angstrom and about 1000 angstroms.
 8. The method of claim 1, further comprising iteratively repeating the forming and thermally annealing steps to form a carbon layer that has a thickness between about 200 angstroms and about 800 angstroms.
 9. The method of claim 1, wherein forming the carbon layer comprises depositing the carbon-based reversible resistance switching material using a plasma enhanced chemical vapor deposition technique.
 10. The method of claim 1, further comprising forming a steering element coupled to the carbon layer.
 11. The method of claim 10, wherein the steering element comprises a p-n or p-i-n diode.
 12. The method of claim 10, wherein the steering element comprises a polycrystalline diode.
 13. The method of claim 1, further comprising prior to forming an additional layer above the carbon layer, treating the carbon layer with ultraviolet (“UV”) radiation.
 14. A memory cell formed according to the method of claim
 1. 15. A method of forming a memory cell, the method comprising: forming a layer of a carbon-based reversible resistance switching material above a substrate; and prior to forming an additional layer above the carbon layer, treating the carbon layer with ultraviolet (“UV”) radiation.
 16. The method of claim 15, wherein the UV radiation has a wavelength between about 230 nanometers and about 400 nanometers.
 17. The method of claim 15, wherein the UV treatment step comprises exposing the carbon layer to UV radiation for about 1 second to about 60 minutes.
 18. The method of claim 15, wherein the UV treatment step comprises exposing the carbon layer to UV radiation at a pressure between about 30 mT and about 760 T.
 19. The method of claim 16, further comprising doping the carbon layer by performing the UV treatment step in an environment that includes a gas comprising any of nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, helium, or xenon.
 20. The method of claim 16, wherein the carbon layer comprises amorphous carbon.
 21. The method of claim 20, wherein the amorphous carbon contains nanocrystalline graphene.
 22. The method of claim 15, wherein the carbon layer has a thickness is between about 1 angstrom and about 1000 angstroms.
 23. The method of claim 15, wherein the carbon layer has a thickness between about 200 angstroms and about 800 angstroms.
 24. The method of claim 15, wherein forming the carbon layer comprises depositing the carbon-based reversible resistance switching material using a plasma enhanced chemical vapor deposition technique.
 25. The method of claim 15, further comprising forming a steering element coupled to the carbon layer.
 26. The method of claim 25, wherein the steering element comprises a p-n or p-i-n diode.
 27. The method of claim 25, wherein the steering element comprises a polycrystalline diode.
 28. The method of claim 15, further comprising prior to forming an additional layer above the carbon layer, thermally annealing the carbon layer.
 29. A memory cell formed according to the method of claim
 15. 30. A method of forming a memory cell, the method comprising: forming a layer of a carbon-based reversible resistance switching material above a substrate; and prior to forming an additional layer above the carbon layer, treating the carbon layer with ultraviolet (“UV”) radiation and then thermally annealing the carbon layer.
 31. The method of claim 30, wherein thermally annealing comprises thermally annealing at a temperature of between about 500° C. and about 1200° C.
 32. The method of claim 31, wherein thermally annealing comprises thermally annealing for about 1 minute to about 300 minutes.
 33. The method of claim 30, wherein the UV radiation has a wavelength between about 230 nanometers and about 400 nanometers.
 34. The method of claim 30, wherein the UV treatment step comprises exposing the carbon layer to UV radiation for about 1 second to about 60 minutes.
 35. The method of claim 30, wherein the UV treatment step comprises exposing the carbon layer to UV radiation at a pressure between about 30 mT and about 760 T.
 36. The method of claim 30, further comprising doping the carbon layer by performing the UV treatment step and/or the thermal anneal step in an environment that includes a gas comprising any of nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, helium, or xenon.
 37. The method of claim 30, wherein the carbon layer comprises amorphous carbon.
 38. The method of claim 37, wherein the amorphous carbon contains nanocrystalline graphene.
 39. The method of claim 30, wherein the carbon layer has a thickness between about 1 angstrom and about 1000 angstroms.
 40. The method of claim 30, wherein the carbon layer has a thickness between about 200 angstroms and about 800 angstroms.
 41. The method of claim 30, wherein forming the carbon layer comprises depositing the carbon-based reversible resistance switching material using a plasma enhanced chemical vapor deposition technique.
 42. The method of claim 30, further comprising forming a steering element coupled to the carbon layer.
 43. The method of claim 42, wherein the steering element comprises a p-n or p-i-n diode.
 44. The method of claim 42, wherein the steering element comprises a polycrystalline diode.
 45. A memory cell formed according to the method of claim
 30. 46. A method of forming a memory cell, the method comprising: forming a layer of a carbon-based reversible resistance switching material above a substrate; and prior to forming an additional layer above the carbon layer, thermally annealing the carbon layer in an environment that includes a gas comprising any of nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, helium, or xenon.
 47. The method of claim 46, wherein thermally annealing comprises thermally annealing at a temperature of between about 500° C. and about 1200° C.
 48. The method of claim 47, wherein thermally annealing comprises thermally annealing for about 1 minute to about 300 minutes.
 49. The method of claim 46, wherein the carbon layer comprises amorphous carbon.
 50. The method of claim 49, wherein the amorphous carbon contains nanocrystalline graphene.
 51. The method of claim 46, wherein the carbon layer has a thickness between about 1 angstrom and about 1000 angstroms.
 52. The method of claim 46, wherein the carbon layer has a thickness between about 200 angstroms and about 800 angstroms.
 53. The method of claim 46, wherein forming the carbon layer comprises depositing the carbon-based reversible resistance switching material using a plasma enhanced chemical vapor deposition technique.
 54. The method of claim 46, further comprising forming a steering element coupled to the carbon layer.
 55. The method of claim 54, wherein the steering element comprises a p-n or p-i-n diode.
 56. The method of claim 54, wherein the steering element comprises a polycrystalline diode.
 57. The method of claim 46, further comprising prior to forming an additional layer above the carbon layer, treating the carbon layer with ultraviolet (“UV”) radiation.
 58. A memory cell formed according to the method of claim
 46. 